Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [74]
Despite our frequent and long-standing searches in the winter woods of Maine, there were many species in my father’s collection that we never found in hibernation. Perhaps they have specialized hibernation sites that we didn’t know about. We found some species in hibernation that we never saw during the summer, so that our winter searches served as a way to enlarge our survey of the diversity of species.
Virtually nothing was known then of the physiological and behavioral mechanisms insects use to survive winter. But thanks to numerous researchers and a huge amount of work in recent decades, we now know that even in any one locality the different insect species exhibit a diversity of winter survival tactics. As previously indicated in the overwintering moth larvae that the kinglets feed on, a few species have amazingly evolved to survive being frozen solid, sometimes at temperatures much lower than those that would kill freezing-tolerant frogs. Some seek special shelter. A tiny number migrate. Many species avoid freezing by physiological mechanisms that lower the freezing point of their tissues. No one strategy is best since each has envolved under a different set of constraints and opportunities. The widest variety of ways of surviving winter is exhibited by the Lepidoptera (moths and butterflies), and I turn to them for a comparative view to explore the range of possibilities. They are also the best studied so far.
Spring azure pupa and adult.
Mourning cloak butterfly.
From northern New England and all the way across Canada, Europe, and Alaska, one of those which stays and overwinters as an adult is the mourning cloak butterfly (Nymphalis antiopa). In the summer the massed crowds of their black-and-red spiny caterpillars commonly feed on willow. Two generations are commonly produced per year, and second-brood caterpillars that I’ve reared in the fall always pupate and produce their adults before winter. The mourning cloaks, like many other butterflies in their family, the Nymphalidae, overwinter only in the adult stage. Mourning cloaks hibernate in hollow trees and these long-lived (in excess of 10 months) butterflies feed on fermenting tree sap such as a sapsucker licks. I never fail to see them in early spring as they bask with open wings in sunshine usually long before all the snow has melted.
Probably my favorite early spring butterfly is the spring azure (Celastrina ladon). Those brilliant blue mites that, like winged jewels, flutter over the just-emerging brown earth after the long winter. Females mate on the first day after emergence, lay their eggs on the second, and rarely live beyond the third. The larvae are tended by ants, and the pupae then spend most of the summer and then the winter in diapause.
In all overwintering insects only one life stage is adapted to survive winter. In the eastern tent caterpillar moth (Malacosomia americanum) that stage is the egg. The moth lays her eggs all in one bunch in a ring around a cherry or apple twig where they are encased in a foam that hardens so that the eggs become solidly attached to the twig. The eggs are exposed to the winter’s lowest air temperatures but what shields them from the cold is glycerol, an antifreeze chemical that used to be commonly poured into car radiators in the fall. In early spring, the glycerol is depleted from the eggs and the larvae